1. Field of the Invention
This invention relates to a two-zone hydrotreating process for middle distillate petroleum fractions. More particularly, this invention relates to an improved process for removing sulfur and nitrogen compounds and hydrogenating olefins and polynuclear aromatics present in middle distillate petroleum feed stocks. Still more particularly, this invention relates to reducing hydrogen consumption in hydrotreating processes by first passing steam and the petroleum feed to a steam hydroconversion zone to produce hydrogen therein and wherein the hydrogen produced reacts in situ in said zone to partially saturate and desulfurize said feed before same is passed to the hydrotreating zone, thereby greatly reducing hydrogen requirements for the hydrotreating zone.
2. Description of the Prior Art
The petroleum industry is increasingly turning to coal, tar sands and heavy crudes as sources for future raw materials. Feed stocks derived from these heavy materials are more deficient in hydrogen and higher in sulfur and nitrogen than feed stocks derived from more conventional crude oils. Such feed stocks are also referred to as being dirtier. These feeds therefore require a considerable amount of upgrading in order to obtain usable products therefrom, such upgrading or refining generally being accomplished by hydrotreating which includes reactions such as hydrodesulfurization to remove sulfur compounds, hydrogenating to saturate olefins and condensed aromatics and also hydrodenitrogenation. Hydrotreating requires large volumes of hydrogen and consequently incurs relatively high processing costs. It is therefore desirable to develop hydrotreating processes with reduced hydrogen consumption.
One way of obtaining hydrogen is via steam reforming. Steam reforming is well known to those familiar in the art as a process for producing hydrogen or hydrogen containing gas mixtures by converting hydrocarbons with steam. Hydrocarbons present in a feed react with steam to form carbon monoxide and hydrogen in a gasification reaction. The carbon monoxide is then reduced to a low level by a water gas shift reaction, which also produces more hydrogen. The two reactions combine in steam reforming as illustrated by the following equations. Steam reforming: CnH.sub.2 n+2nH.sub.2 O 3nH.sub.2 +nCO.sub.2 =a)gasification: CnH.sub.2 n+nH.sub.2 O 2nH.sub.2 +nCO +b)watergasshift: nCO+nH.sub.2 O nH.sub.2 +nCO.sub.2
Unfortunately, most catalysts used for steam reforming are not resistant to sulfur and, consequently, their catalytic activity rapidly diminishes to an unacceptably low level in the presence of sulfur-containing hydrocarbons. Further, many of the well known sulfur tolerant hydrogenation catalysts are deactivated in the presence of steam and are therefore unsuitable in steam reforming processes. The prior art teaches that if one desires to hydrotreat a sulfur-containing feed and make use of steam reforming as a source of hydrogen with the sulfur-containing feed, the feed must be passed to a first zone wherein the sulfur is removed, followed by steam reforming in a second zone to produce hydrogen from the desulfurized feed, followed by passing the feed, along with the hydrogen produced in the steam reforming zone to a third zone wherein the olefinic and polynuclear aromatic components in the feed are saturated via catalytic hydrogenation.
It would therefore be a significant improvement to the art if one could employ steam hydroconversion in a single zone, with a sulfur-containing feed, to produce hydrogen and utilize the hydrogen so produced to in situ saturate at least a portion of the olefins and polynuclear aromatics in the feed in said zone, as well as removing at least some of the sulfur therefrom to produce a semi-refined or hydrotreated feed, and then pass same to a hydrotreating zone to saturate the remaining olefins and polynuclear aromatics along with the hydrodesulfurization and hydrodenitrogenation of the sulfur and nitrogen compounds contained therein. This would greatly reduce the hydrogen requirements for the hydrotreating zone.